NASA's Mars rover, Curiosity, has discovered gravel once carried by the waters of an ancient stream that "ran vigorously" through the area.

Scientists had previously found other evidence of the one-time presence of water on the Red Planet, but the US space agency says this is the first time stream-bed gravels have been discovered.

Project scientist John Grotzinger says the rocky outcrop, called Hottah, "looks like someone jack-hammered up a slab of city sidewalk, but it's really a tilted block of an ancient stream bed".

Curiosity also investigated a second outcrop, called "Link".

The pictures transmitted by Curiosity show the pebbles have been cemented into layers of conglomerate rock at a site between the north rim of the Gale Crater and the base of Mount Sharp - where Curiosity is heading.

Curiosity science co-investigator Rebecca Williams says the sizes and the shapes of the rocks give an idea of the speed and the depth of the stream.

"The shapes tell you they were transported, and the sizes tell you they couldn't be transported by wind," she said.

"They were transported by water flow."

The scientists estimate the water was moving briskly at one metre per second and ran somewhere between ankle and hip deep.

The high number of channels between the rim and the newly discovered rock bed suggests the stream was not a one-time occurrence but that many streams flowed or repeated over a long time.

Curiosity is on a two-year mission to investigate whether it is possible to live on Mars and to learn whether conditions there might have been able to support life in the past.

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